1. Field of the Invention
This invention refers to a method for the reduction of the sag and/or of the varying stress profile of a flattened tubular film web produced by a blown film process and made of thermo-plastic synthetic resin, transversely to its longitudinal direction.
2. Description of Prior Art
During their production, after the inflation to form a tubular film bubble, tubular films are drawn thereoff by a pair of pinch rolls which are preceded by flattening plates extending wedge-shaped to each other, which converge in direction towards the roll gap of the pair of pinch rolls. A result of this kind of flattening of the tubular film web drawn off from the tubular film bubble is that the laterally folded edge sections of the tubular film web directly pass into the roll gap of the pair of pinch rolls, whereas the web portions lying between the folded edge sections of the web slide over the inner sides of the flattening plates with an unavoidable friction and, starting from opposite sleeve lines of the film web, are progressively pressed in a flat form in which then the wall parts of the web, lying flat on top of each other, pass through the draw-off roll gap. The result of this kind of flattening of the tubular film web drawn off the tubular film bubble is that in the flat state the center parts of the web portions lying one on top of the other have been stretched in a longitudinal direction to a larger degree than the folded edge sections which without a previous sliding on the flattening plates have directly passed into the roll gap of the pair of draw-off rolls. By said different stretching of the web portions of the tubular film web, lying on top of each other, with respect to the edge section the web has a certain sag in its center section, which becomes visible when the flattened web is observed which is loosely supported in a horizontal plane. The sag can also be observed when the flattened tubular film web is separated along its width into longitudinal strips of identical width, and weights of identical size are hanged onto the individual longitudinal strips. Due to the different lengths of the web sections, the weights then show the form of a curved line.
Due to this different stretching of the tubular film web along its cross-section during its production, different tensions in the web are obtained transversely to its longitudinal direction.
If such a tubular film web were e.g. to be cut along its center line, the two half tubular webs would be deflected to the outside, since the tension at the outside is larger than the one in the center section, due to which the half tubular webs would show a curved form. Such curvings of webs, however, are undesirable with a view to the further processing. If such webs were to be printed in printing machines, decisive difficulties would arise, since the register and the corner mark (format lengths) could not be exactly met.